In an optical fiber communication adopting WDM (Wavelength Division Multiplexing), a demodulator including a delay interferometer demodulates an optical signal modulated by differential phase shift keying. In this case, a device for adjusting a phase is needed because phase difference is changed according to a wavelength of a received signal. For example, there is known a technology in which a phase is adjusted by heating a medium on an optical path and adjusting refraction index of the medium.
On the other hand, a polarized wave in a transmission path of an optical communication fluctuates at random. Therefore, it is preferable that a receiving device keeps property with respect to each polarized wave, in order to restrain reduction of extinction ratio. However, the property may be degraded when appropriate phase shift amount of a demodulator is different with respect to each polarized wave. Japanese Patent Application Publication No. 2005-80304 discloses a technology using a delay interferometer with use of a particular beam splitter of which phase shift amount of each polarized wave is small.